Anti-lock brake control system

ABSTRACT

An electronic control for an anti-lock brake control system provides different deceleration threshold reference signal levels for high and low coefficient surfaces. A coefficient select circuit is responsive to wheel acceleration during the period of wheel speed recovery in an anti-lock cycle and selects for use during the subsequent cycle one of the threshold values in accordance with the degree of wheel acceleration. Another circuit senses wheel hop during braking to further increase the threshold during wheel hop to desensitize the control to undesirable transients.

United States Patent 1 1 Harned Feb. 20, 1973 541 ANTI-LOCK BRAKECONTROL 3,578,819 1971 Atkins ..303 21 P SYSTEM 3,499,689 3/1970 Carp eta1 ..303/21 P 3,544,172 l/l970 Howard et al. ....303/21 BE Inventor! Jllamed, Grosse Pomte 3,545,819 12/1970 Gatfney etal. .303/21 R Woods,Mich. Prima Examiner-Tr ve M. Blix l t C t Yg [73] Asslgnee gil s; r orsorpom Assistant Examiner-Stephen G. Kunin Attorney-Jean L. Carpenter,Paul Fitzpatrick and [22] Filed: March 15, 1971 Warren D. Hill [21]Appl. No.: 124,336 ABSTRACT An electronic control for an anti-lock brakecontrol [52] Cl "303/21 188/181 system provides different decelerationthreshold Int Cl 86m 8/12 reference signal levels for high and lowcoefficient surfaces. A coemciem Select circuit is responsive to [58] ofSearch 188/ 303/ 3 1 wheel acceleration during the period of wheel speedrecove in an anti-lock c cle and selects for use durh ry b l y f h h h1d 1 mg t e su sequent cyc e one o t e t res o va ues 1n [56] ReferencesC'ted accordance with the degree of wheel acceleration. UNITED STATESPATENTS Another circuit senses wheel hop during braking to furtherincrease the threshold durmg wheel hop to 3,604,761 1971 Okamoto et BE Xdesensitize the control to undesirable transients. 3,604,762 9/1971 Andoet al. ..303/2l BE 3,606,490 9/ 1971 Ando ..303/2l BE 6 Claims, 4Drawing Figures r12 [TACHOMETER E w bl? 26 WHEEL 28 INFORMATION T il/253 {x vAliiEogcNTRoL CIRCUIT 28 INTEGRATOR 32212 7o 20 24 cmcun 70 72To ATMOSPHERE WHEEL HOP SUPPRESSICN cmcun CIRCUIT RESET COEFFICIENTVACUUM s EEB 1 76 SOURCE r58 W V, w... ,.i1

11155575555! I g E RECOVERY 7? CIRCUIT 53 ALVE CONTROL LOGIC TOAMOSPHERE VACUUM SOURCE f58 QC HYSTERESIS SWITCH sum 10F 2 DRIVERELATIVE VELOCITY INTECRATOR LOCIC CIRCUIT WHEEL HOP CIRCUIT 6&

CIRCUIT RESET SELECT CIRCUIT RECOVERY CIRC IT INERTIA WHEEL VELOCITYREFERENCE VEHICLE WHEEL THRESHOLD SUPPRESSION WHEEL SPEED COEFFICIENTVEHICLE VELOCITY PAIENIEDFEBZOIQYS WHEEL CIRCUIT TACHOMETER INFORMATIONommam WHEEL VELOCITY TIME umnwmmma PATENTEB FEBZO 197s SHEET 2 OF 2INVENTOR. fa/m L. Harned ANTI-LOCK BRAKE CONTROL SYSTEM This inventionrelates to anti-lock brake control systems and in particular to such asystem which is adapted to the coefficient of friction between thebraked wheel and the road surface.

It has previously been proposed as in the patent application of John L.I-Iarned, Ser. No. 754,252, now U.S. Pat. No. 3,554,612 to provide ananti-lock brake control which senses incipient lock-up by comparing thedeceleration of a braked wheel with a predetermined referencedeceleration. In such systems it is necessary to select a referencedeceleration which is suitable for all vehicle operating conditions.However, a deceleration reference which is optimum for a dry road havinga high coefficient of friction is not usually the best value for use onan icy road having a low coefficient of friction. It is thereforedesirable to provide a reference deceleration which varies according tothe coefficient of friction between the braked wheel and the roadsurface. It has been found that the value of acceleration of a brakedwheel during the wheel speed recovery phase when the brake pressure hasbeen relieved can be used as a measure of the coefficient of frictionand that the deceleration reference may be varied according to thatacceleration to provide an anti-lock brake control system adaptive toroad conditions.

It is therefore a general object of the invention to provide ananti-lock brake control having a reference signal variable in accordancewith wheel acceleration of a previous anti-lock brake control cycle.

A further'object of the invention is to provide an anti-lock brakecontrol system having high and low deceleration reference thresholds anda circuit for selecting the appropriate threshold according to thecoefficient of friction of the road surface.

Another object is to provide an anti-lock brake control having a circuitfor sensing transient wheel hop signals and momentarily increasing thedeceleration reference threshold to suppress their effect.

The invention is carried out by providing a circuit responsive torotation of a braked wheel for sensing incipient wheel lock-up andcyclically relieving and reapplying brake pressure to prevent wheellock-up wherein a deceleration threshold signal has a value predicatedon a measure of wheel deceleration during a previous brake releaseperiod. In particular, the invention contemplates that a highdeceleration threshold reference will be applied when the braked wheelexperiences a high acceleration during the previous release cycle and alow threshold when the wheel experiences a low acceleration during theprevious cycle.

The invention is further carried out by providing a control circuitresponsive to braked wheel acceleration for sensing incipient wheellock-up and including a circuit for sensing transient wheel hopacceleration signals and for increasing the deceleration referencethreshold to render the control circuit less sensitive to the wheel hopsignals.

The above and other advantages will be made more apparent from thefollowing specification taken in conjunction with the accompanyingdrawings wherein like reference numerals refer to like parts and whereinFIG. 1 is a block diagram of an anti-lock brake control system coupledwith a schematic diagram of a hydraulic brake system with a brakepressure modulator according to the invention;

FIG. 2 is a schematic diagram of the control circuit of FIG. 1 accordingto the invention;

FIG. 3 is a graphical representation of vehicle speed, wheel speed andbrake pressure to illustrate the operation of the control systemaccording to the invention as applied to a high coefficient surface;and,

FIG. 4 is a graphical representation of vehicle speed and wheel speed toillustrate the operation of the control system as applied to a lowcoefficient surface.

The system in which this invention is utilized follows the wellestablished principle of sensing incipient wheel lock-up when brakepressure is applied to vehicle brakes, then relieving the brake pressureuntil the wheel accelerates enough to be out of danger of locking andthen reapplying the brake pressure. This cycle is repeated as necessaryto achieve the desired braking action. This system is particularly welladapted to practice the control principle of the extremal type in whichbrake pressure sufficient to cause a substantially increased wheel slipis released to permit wheel acceleration and consequent decreasing wheelslip. The brake pressure is then controlled to permit wheel accelerationand therefore a decrease in wheel slip while maintaining a brake torqueon the wheel until the wheel acceleration ceases. The brake applypressure then again is increased to cause wheel deceleration. Thisextremal type of control is more fully set forth in the US. Pat. No.3,441,320 to D. M. Flory.

The system further employs an improved version of the Inertia WheelVelocity Reference Principle which is fully set forth in the aforesaidSer. No. 754,252. Stated briefly, the principle involves measuring thevelocity of a braked wheel and subtracting therefrom a simulated vehiclevelocity (or optimum wheel velocity) and utilizing the difference as acriterion of when the wheel brake should be released and reapplied formost effective control.

Referring to FIG. 1 for a further description of the system, a vehiclewheel drive 10 is provided as an information source for the system. Thevehicle wheel drive 10 may be a wheel per se, a propeller shaft drivinga plurality of wheels or any other vehicle member having a velocity orrotation proportional to wheel velocity. A

tachometer 12 such as a well-known toothed wheel variable reluctanceelectromagnetic transducer is driven by the wheel drive 10 and providesan alternating signal having a frequency proportional to the wheel speedon line 14 and leading to a wheel information circuit 16. The wheelinformation circuit prepares a DC voltage or current proportional towheel acceleration on lines 18 and 20. The wheel acceleration signal online 18 is compared with an appropriate threshold signal on line 22which is prepared by a threshold logic circuit 24 and the differencebetween the signals is integrated by a relative velocity integrator 26to produce a relative velocity error signal on line 28, which signal isutilized by a hysteresis switch 30 to signal via line 32 a valve controllogic circuit 34 to release brake pressure.

A brake pressure modulating system comprises a combination pneumatic andhydraulic system. The pneumatic portion comprises a tube 36 vented toatmosphere at one end and connected at the other end to a vacuum source38 which may be the vehicle engine manifold. The tube 36 contains inseries a restricting orifice 40 near the vent to atmosphere, a pressureaccumulator 42, a normally closed electrically operated release valve 44controlled by an output signal on the conductor 46 from the valvecontrol logic circuit 34, and a second restricting orifice 48. A branchtube 50 is connected to the tube 36 at a point between the valve 44 andthe orifice 48 and leads to a pressure modulator I 52. Another pneumatictube 54 connects the modulator S2 with the vacuum source 38. Thehydraulic portion of the system includes a manually operatedconventional master cylinder 56 for applying brake pressure toconventional vehicle brakes 58 through a first supply line 60 tomodulator 52 and a second supply line 62. The structure and operation ofthe modulator 34 is fully described in the above-mentioned Flory patentand no further description herein is necessary except to point out itsfunction.

Normally, with valve 44 closed, vacuum pressure from the source 38 isapplied through the tubes 50 and 54 to both sides of the modulator 52and in this condition, the modulator transfers the brake pressure from Vthe master cylinder 56 to the brakes 58 without any modulation. However,when the valve 44 is opened to supply air pressure from the atmosphereto the modulator 52 through the tubes 36 and 50, the modulator willfirst isolate the brakes 58 from the master cylinder 56 and will thenrelieve pressure from the brakes 58 to a degree determined by the amountof air pressure supplied through the tube 50. The function of theaccumulator 42 is to permit a rapid release of brake pressure for ashort time after the valve 44 is opened. Thereafter brake pressure willdecrease more slowly as determined by the flow of air through theorifice 40. When the valve 44 is closed, the brake pressure increasestoward master cylinder pressure at a controlled rate deter-,

mined by the air flow through the orifice 48.

The threshold logic circuit 24 senses the operating characteristics ofthe system and selects the appropriate threshold to be biased againstthe incoming wheel deceleration signal. Before brake pedal application,the circuit reset 64 signals on line 65 a 12g threshold. This masks anyrandom noise signal from momentarily triggering the anti-lock control,for example, when controlled wheels have been overspeed during vehicleacceleration. The remaining thresholds are applied depending onoperating mode, road surface coefficient, and impending wheel hop duringrough road operation.

The coefficient select circuit 66 senses the wheel acceleration signalon line 20 to determine whether the wheel speed recovers in excess of4gs. If it does, then a 2g threshold is applied via line 67 to thethreshold logic circuit 24 during the next brake apply cycle. Otherwise,a lg rate is selected. The 4g wheel speed recovery rate discriminatesbetween most road surfaces and the very slippery icy surfaces whichrequire a lower threshold for proper operation. The coefiicient selectcircuit 66 also senses the operation of the hysteresis switch 30 via theline 68 to reset the lg threshold whenever the hysteresis switch callsfor a brake release. The coefficient select circuit 66 is also actuatedby the circuit reset 64 via line 69 to further increase the thresholdwhen the brake pedal is not actuated.

A wheel hop suppression circuit 70 senses wheel accelerations in thewheel hop frequency range by filtering the acceleration signal on line20 with a bandpass filter. When this circuit senses an acceleration of2gs or more at the wheel hop frequency, then it signals the thresholdlogic circuit 24 via line 72 to apply an additional threshold of 1.5 gsduring the brake apply mode.

The hysteresis switch also unlocks the wheel hop suppression circuit vialine 68 to insure that the 1.5g threshold is off during release. No morethan lg is applied to the relative velocity integrator 26 duringrelease.

The wheel speed recovery circuit 74 is responsive to the accelerationsignal and provides the operating sense to appropriately control thevalve control logic 34 via the line 76 during the wheel speed recoveryphase of the cycle, which controls the hold mode and the 2g applytechnique. The 2g apply technique allows brake pressure to be reappliedduring wheel speed recovery whenever the acceleration of the wheelexceeds approximately 2gs. The wheel speed recovery circuit alsodisables the hysteresis switch 30 via line 77 to prevent brakeapplication when the wheel acceleration exceeds 0.5g.

FIG. 2 illustrates the details of the control circuit. Wheel InformationCircuit The wheel information circuit 16 has as an input fromthetachometer 12 on line 14 a train of pulses having a frequencyproportional to the sensed wheel speed. The pulses are fed into a highgain squaring amplifier 80 having an input resistor 82 and a feedbackresistor 84 to produce an output train of square pulses with constantamplitude and edge slope. A differentiator comprises a capacitor 86, aresistor 88 and a diode 90 in series with the output of the amplifier 80and a diode 92 between the capacitor 86 and ground, and produces anegative pulse of constant area for every square pulse input. Theduration of the pulse is less than the period of the input pulses atmaximum vehicle speed. This prevents an overlap of pulses which wouldintroduce non-linearities. The negative pulses from the dif ferentiatorare then filtered by an active first order low pass filter whichcomprises an operational amplifier 94 having a feedback capacitor 96 anda feedback resistor 98. A feedback diode 100 in conjunction with abiasing voltage divider network including resistors 102 and 104, sets alow speed threshold that must be overcome by the negative pulses beforea positive voltage will appear at the output of the amplifier 94. Thisprovides a low speed cutout feature for the circuit. The resistor 104 isconnected at opposite ends to a 7.5v and +7 .5v 7

DC power supply. The output of the amplifier 94 is a positive voltageproportional to wheel speed. This voltage is then differentiated by twocircuits. The first is a non-inverting passive differentiator comprisinga series connected capacitor 106 and resistor 107 which produces acurrent on line 18 proportional to wheel acceleration. The seconddifferentiator is also a passive differentiator comprising capacitor 110and a resistor 112 serially connected between the output of theamplifier 94 and the input of a non-inverting operational amplifier 114.A low pass filter comprises feedback capacitor 116 and feedback resistor118 across the amplifier 114 to smooth out the ripple. Threshold LogicCircuit The threshold logic circuit 24 includes an array of inputresistors 120, 122, 124 and 126, each having one end connected to thecommon line 22. The other ends of the resistors are connected to theoutputs of the wheel hop suppression circuit 70, the coefficient selectcircuit 66, the circuit reset 64 and the +7.5v DC power supplyrespectively. The resistor values are chosen to provide threshold biascurrents to line 22 according to the desired circuit functions asrecited above. The resistor 126 provides the minimum decelerationthreshold of 1 g. The other thresholds when applied are additive andwill increase the threshold according to their respective values.Relative Velocity Integrator The relative velocity integrator circuit 26senses the wheel acceleration signal on line 18 and the threshold biason line 22. The integrator includes an operational amplifier 128 havinga feedback circuit including a gain resistor 130 in series with anintegrating capacitor 132, a resistor 134 between ground and thejunction of the resistor 130 and capacitor 132, and a diode 136 acrossthe resistor 130. These components give the amplifier a dual integrationgain. A high gain exists when the amplifier output is negative and thediode 136 is blocking. However, as the amplifier output moves into itspositive range, the diode 136 conducts to shunt the gain resistor 130 tolower the gain. This feature offers a sensitive range for triggeringsystem switch points for control purposes and a lower gain range forstoring excessive relative wheel speed buildup. A pair of oppositelypoled Zener diodes 137 and 140 are serially connected across theamplifier 128 to limit the amplifiers output short of its saturationvalues. In operation, when the system is not controlling, that is, whenthe current on line 18 flowing from the amplifier 128 does not exceedthe threshold current on line 22, the output of the amplifier 128 isslightly above negative saturation. As the wheel deceleration increasesto a value exceeding the threshold, then the output of the amplifier 128increases, i.e., becomes less negative to signify the velocity error orthe difference between the wheel velocity and the simulated vehiclevelocity. Hysteresis Switch The hysteresis switch 30 is operated by theoutput of the relative velocity integrator 26. This switch includeslogic NOR gates as does much of the remainder of the circuit to bedescribed. The gates switch at, say, 0.7v, so that when both inputs to aNOR gate are low (below 0.7V), then the gate output must be high (above0.7V). If either or both inputs to a NOR gate are high, then the outputwill be low. NOR gates 142 and 144 are connected to form a set-resetflip-flop. The flip-flops inputs a and 0 respond to different relativevelocity output levels of the amplifier 128. Input a to NOR gate 142 isconnected through a resistor 146 to the output of the amplifier 128 online 28. An NPN transistor 148 has its base connected through a resistor150 to the line 28 and its emitter is connected to the midpoint of avoltage divider comprising resistors 152 connected between a 7.5v DCsource and ground. The collector is connected through series resistors154 and 156 to +7.5v. The junction point of resistors 154 and 156 forman input 2 to a NOR gate 158 which has its other input grounded andtherefore acts simply as an inverter. The output of the gate 158comprises an input 1' to a NOR gate 160 which has its other input signalh from the wheel speed recovery circuit 74. The output of the NOR gate160 forms the input c to the NOR gate 144 while the other input of theNOR gate 144 is the signal d from the output of the NOR gate 142. Tocomplete the flip-flop circuit, the output of the NOR gate 144 comprisesan input b of NOR gate 142. Initially, when the voltage on line 28 isnegative, transistor 148 is off and input 0 to gate 144 is high,assuming that the input h to gate is low, as it will be during periodsof wheel deceleration. Thus signal b will be low and, since input a isalso low, signal 6 will be high. Circuit values areso selected that whena velocity error of one-half ft. per second is accumulated by therelative velocity integrator, the transistor 148 starts to conduct sothat signal z goes low, signal i goes high and signal 0 goes low.However, since signal dis high, the gate 144 will not change state untilinput a turns high. Input a remains low until 4 ft. per second velocityerror has been accumulated by the relative velocity integrator, at whichtime the line 28 becomes positive and input a will turn high. Thisswitches signal d low which allows b to turn high. The flip-flop thuschanges state and will not change back until the voltage on line 28returns to the one-half ft. I

per second switch point that turns off transistor 148 to switch input cto high. Thus, the hysteresis switchs output d remains high until a 4ft. per second relative velocity is reached, then it switches low untilthe velocity error diminishes to less than one-half ft. per second. Eventhen the output d will remain low until the signal h goes low whereupond switches high. The output d is connected via line 32 to the valvecontrol logic 34. Valve Control Logic The valve control logic 34consists of a buffer amplifier 162 which is an inverting currentamplifier having a .single input supplied by a diode 164 connected tosignal d on the line 32 and by a diode 166 connected with a signal j online 76 from the wheel speed recovery circuit. If both inputs d and jare low, then the amplifier 162 will be turned on to energize thenormally closed solenoid valve 44 and release brake pressure. If,however, either the input d or j is turned high, then the amplifier 162will be turned off to close the solenoid valve. The circuits describedto this point relative to FIG. 2 are sufficient to control a basicanti-lock system. How- 'ever, to insure satisfactory system performanceunder virtually all possible operating conditions, the remainingcircuits including the wheel speed recovery circuit 74, the wheel hopsuppression circuit 70, the coefficient select circuit 66, and thecircuit reset 64 are employed. Wheel Speed Recovery Circuit The wheelspeed recovery circuit 74 accomplishes two objectives: (1) It reducesthe complexity and cost of the hydraulic modulator by making a hold modesolenoid valve unnecessary and, (2) it reduces the total responsecharacteristics necessary for good anti-lock control performance byinstituting brake apply during the wheel speed recovery when the wheelaccelerates in excess of 2gs. The wheel recovery logic circuiteliminates the need for a separate hold valve by replacing its functionwith a rapid switching of the release valve 44 between release and applyto modulate a nearly constant brake pressure.

NOR gates 168 and 170 are connected as a set-reset flip-flop circuitwhich constitutes a hold switch. Signal d from the hysteresis circuitforms the reset input and is applied to gate 168. Input h is applied tothe gate 170 for the set input and is produced by a voltage leveldetector comprising a diode 172 connected between the line 20 and theinput h so that when the positive acceleration signal is below 0.5g, thediode 172 does not conduct and the input h is low. However, when theacceleration exceeds that value, h becomes high. The output f of theflip-flop circuit and the input h comprise inputs to a NOR gate 174. Theoutput g of the gate 174 provides an input to a multivibrator whichincludes NOR gates 176 and 178. The output j of gate 176 is connectedthrough a capacitor 180 to an input n of the I gate 178. The input n isalso connected to ground through a resistor 182. Similarly, the output kof the gate 178 is connected through a capacitor 184 to an input m ofthe gate 176, which input is also connected to ground through a resistor186. The other input 1 of the gate 178 is connected through a Zenerdiode 188 to the diode 172. The Zener diode is selected to break down at1.5g's so that it acts as a second voltage level detector. Thus, whenwheel acceleration exceeds 2gs, the voltage on line 20 will besufficient to cause both the diode 172 and the Zener diode 188 toconduct to switch the input x to a high level, whereas at lower valuesof acceleration, the Zener diode 188 will not conduct and the input xwill be at a low level. The output j of the gate 176 is connectedthrough an output resistor 190 to the line 76.

The multivibrator has three operating modes. When the input g is high,the output j is low to enable a brake decay timing by the capacitors 180and 184 and resistors 182 and 186. Since in is normally low and g islow, output j will be high turning signal n high momentarily. As thecharge on the capacitor 180 decays, n will become low and signal k willswitch high to momentarily turn the signal in high which in turn turnsthe output j low until the capacitor 186 discharges and in turns to itslow level causing j to switch high again to cause repetition of thecycle.

In operation of the wheel speed recovery circuit, prior to brakerelease, the signal a is high causing the signal f to be low. Since theinput h must be low during brake application and wheel deceleration,then the signal e is high. Since the signals f and h are low, then thesignal g will be high and the signal j will be low to thereby permitbrake release when signal d is switched low by the hysteresis switch.However, at brake release when d goes low, the flip-flop circuit doesnot change state since the signal e is high and f remains low. When,however, after brake release the wheel accelerates to a value above0.5gs, the signal h goes high to cause 3 to go low, causing j tooscillate to automatically turn the valve 44 on and off to hold thebrake pressure at a relatively constant value. When, however, theacceleration signal on line 20 exceeds 2gs, the input x goes highcausing the output j to stay high to effect brake application. Shouldthe wheel acceleration then drop below 2gs, the input x will drop to alow level and the output j will return to its oscillating holdcondition. When the wheel recovers sufficient speed that the velocityerror 'drops to one-half ft. per second, the transistor 148 in thehysteresis switch turns off so that signal z at gate 158 goes high andits output i goes low. However, if they wheel is still acceleratingabove a value of 0.5g, the signal It will still be high and the signal cwill remain low so that the flip-flop 142,144 does not change state andthe signal d remains low whereby the wheel speed recovery circuit is thecontrolling agent. When, however, the wheel acceleration drops below a0.5g and signal h goes low, signal 0 will g high permitting signal d togo high to close the valve 44 and reapply the brakes. Wheel HopSuppression Circuit On rough roads the vehicle suspension system isexcited at its wheel hop frequency of about 10 Hz and large wheelacceleration amplitudes are generated, particularly during braking.These large acceleration excursions may cause premature system operationwhich releases brake pressure to assume a value below that' of 10 Hz. AZener diode 198 in series with the filter is selected to conduct thosesignals in excess of 2g to. provide an input u to a NOR gate 200 havingits other input grounded. The output v of the NOR gate 200 provides aninput to a NOR gate 202 having an output w on line 72. The second inputto the NOR gate 202 is the signal bfrom the hysteresis switch 30. Signalb is high during brake release and low at brake apply. This, dur-' ingbrake release, the output w ,is always low. When, however, during brakeapply, the Zener diode 198 senses a transient acceleration in excess of2gs, the input u is high and the signal v will be low so that the signalw will be high. The signal w is connected to the resistor in thethreshold logic circuit 24 to increase the threshold bias by 1.5gs. Thusthe system is made less sensitive to wheel hop signals. When, however,there are no such large wheel hop signals on line 20, the input u of NORgate 200 will be low and the outputw will likewise be low so that thethreshold bias is not effected by the wheel hop suppression circuit.Coefficient Select Circuit On road surfaces where the frictioncoefficient exceeds 0.3, wheel acceleration during wheel speed recoverywill always exceed 4g. On ice covered surfaces, the peak wheelacceleration will always be less than 4g. On surfaces where the frictioncoefficient falls between 0.15 and 0.3, the occurrence of peak wheelaccelerations greater or less than 4g will depend on the wheel lock-up.System extremal stability can be increased by decreasing thedeceleration threshold bias to the relative velocity integrator 26. Theeffect of the coefficient select circuit 66 is to alter the thresholdbias in accordance with the coefficient of friction between the tire androad as reflected in the acceleration signal during wheel speedrecovery. The circuit includes an acceleration signal level detectorcomprising a Zener diode 204 and a diode 206 connected serially to theline 20. These elements in combination conduct at a signal level of 4gsto provide a signal r to one input of a NOR gate 208. That gate, alongwith NOR gate 210, provides a flip-flop circuit such that the output sof the gate 208 serves as an input to the gate 210 and the output 2 ofgate 210 is an input for the gate 208. The other input signal b to gate210 is developed by a capacitor 212 connected from the line 68 carryingsignal b from the hysteresis switch. The input line b is connected toground through a resistor 214. The output s of the flipflop forms oneinput to a NOR gate 216 and the signal b serves as the other input. Theoutput y of the gate 216 is connected via line 67 to the resistor 1 22in the threshold logic circuit 24.

During the brake release phase, b is high and the output y is alwayslow. When the signal b first goes high, at release, the signal isdifferentiated by the capacitor 212 so that the signal b goes highmomentarily to set the flip-flop. That is, when b' goes high, t will golow and since r is always low, at the time of brake release, s will gohigh. During the brake release phase, if the road coefficient is low,the wheel acceleration will not exceed 4gs and the signal r will remainlow so that the signal y will remain low and will therefore notcontribute to the'threshold bias during the subsequent anti lock cycle.If, on the other hand, the coefficient is high, the input r will go highsometime during the wheel recovery phase so that s will go low to resetthe flipflop. That is, t will go high and s will be held low. Then,during the apply phase of the subsequent cycle, when the signal b goeslow, the signal y will go high to increase the threshold bias an amountequivalent to 1 g. The signal y will remain high until brake releasewhereupon signal b goes high to turn y low and b will go highmomentarily to set the flip-flop and return the signal s to a highlevel. A diode 218 connected to an input of the NOR gate 208 carries asignal q. As will be seen, when the vehicle brakes are off, the signal qwill be high to reset the flip-flop to send the signal y high as long asthe brakes are off and during the apply phase of the initial anti-lockcycle.

Circuit Reset The circuit reset 64 comprises a NOR gate 220 having oneinput grounded and the other input connected through voltage dividingresistors 221 and 223 to ground and to conductor 225 carrying the brakelight signal. The output q of the NOR gate 220 is connected to the diode218 as previously described and also via line 65 to the resistor 124 inthe threshold logic circuit 24. The resistor 124 is selected to providea g threshold bias current when q is high. When the vehicle brakes areoff and the vehicle brake lights are not illuminated, the inputs to theNOR gate 220 are low and the output q is high so that the 10g thresholdbias is applied. As mentioned above, when the signal q is high, thesignal y is also high to turn on its corresponding 1g threshold bias.These biases added to the constant lg bias from resistor 126 provide atotal threshold bias of 12gs which effectively desensitizes the relativevelocity integrator 26 so that extraneous wheel decelerationsignals-will not activate the hysteresis switch. When, however, thebrakes are first applied and the brake lights are illuminated, theconductor 225 is energized to provide a high signal to an input of theNOR gate 220 which turns the signal q low and removes the 10g thresholdbias. Then the threshold bias is set at 2gs in readiness for the initialanti-lock cycle.

System Operation The overall operation of the system is best understoodby reference to FIGS. 3 and 4. FIG. 3 is a graph showing vehicle speedon line 222, wheel speed on line 224, reference signal on line 226 andthe brake pressure on line 228. The graph illustrates typical antilockoperation on a high coefficient road surface. The reference signal 226is an imaginary signal not actually existing in the control circuit,however, its slope represents the threshold bias signal. The graphassumes initially no brake pressure is applied and the vehicle speed andwheel speed are constant. At time t brake pressure is first applied andthe vehicle and wheel speeds decrease. The reference signal 226initially has a slope of 2gs as described above. The relative velocityintegrator 26 provides an output on line 28 which is proportionaltovelocity error or the vertical distance between the curves 224 and226. When, at time this distance equals 4 ft. per second, the hysteresisswitch 30 is actuated to switch the signal d low to effect brakerelease, whereupon the brake pressure begins to fall. At the same time,the signal b goes high to turn off the signal y to reduce the thresholdbias thereby changing the reference signal 226 to a slope of lg. At timet the wheel has accelerated to 0.5gs and this acceleration is sensed bydiode 172 in the wheel speed recovery circuit to raise the signal h to ahigh level causing output j to oscillate holding the brake pressureconstant until time Then the wheel acceleration reaches 2gs whereuponthe signal level detector 188 conducts and causes the output j to remainhigh to effect brake reapplication. At time t.,, the wheel accelerationhas decreased to a value below 2gs whereupon the output j againoscillates to hold the brake pressure at a constant value until time t,,when the wheel acceleration has dropped below a 0.5gs level and thewheel is considered to have recovered in speed. Prior to that time, theoutput of the integrator 26 has decreased to one-half ft. per second andthe signal a has gone low. When the signal h goes low at time t,,, thesignal d goes high to cause brake reapplication. Then brake pressurecontinues to increase at a controlled rate until time t, in thesubsequent anti-lock cycle when the relative velocity error againsignifies incipient wheel lock-up and the brake pressure is againreleased. Then the cycles repeat until the vehicle is brought to a stopor the brake pressure is manually released.

During wheel recovery, the wheel acceleration exceeded 4gs signifying ahigh coefficient surface. This acceleration is sensed by the coefficientselect circuit and the 2g threshold is applied during the apply phase ofthe next cycle so that the reference signal 226 is like that of thefirst cycle. This is contrasted to FIG. 4 of the drawing whichillustrates the operation of an anti-lock brake control on an icysurface. The lines 222, 224 and 226 represent the same parameters as inFIG. 3. The brake pressure is not shown in FIG. 4 since it is generallysimilar to that of FIG. 3. As before, in the initial cycle, theimaginary reference signal 226 has a 2g slope until time when the brakesare released and then the slope changes to lg. However, during wheelrecovery, the wheel acceleration does not exceed 4gs so that thecoefficient select circuit 66 is not reset and the signal y remains low.Consequently, for the subsequent cycle, the slope of reference signal226 remains at lg resulting in a more sensitive detection of incipientwheel lock-up so that brake release at time t occurs earlier than if theslope were at 2gs. The system extremal stability is therefore increased.I

The embodiment of the invention described herein is for purposes ofillustration and the scope of the invention is intended to be limitedonly by the following claims.

It is claimed:

1. An anti lock brake control system for a wheeled vehicle adapted tothe coefficient of friction between a braked wheel and the surface onwhich the vehicle is supported, comprising means for generating a firstelectrical signal proportional to acceleration of a braked wheel,

means for producing a deceleration reference electrical signal whichrepresents optimum wheel deceleration for vehicle braking on a givensurface, means responsive to the reference signal and the first signalfor cyclically producing a brake control signal when said first andreference signals attain predetermined relationships, I

brake pressure modulating means responsive to the brake control signalfor cyclically relieving and applying brake pressure, whereby the brakepressure modulating means relieves brake pressure to permit wheel speedrecovery and thereafter reapplies brake pressure, and means responsiveto positive wheel acceleration in each cycle for altering the referencesignal value for the subsequent cycle according to the value of the:positive wheel acceleration during wheel speed recovery, so that thecontrol system is conditioned to the value of wheel acceleration andhence to the coefficient of friction between the wheel and the surface.2. An anti-lock brake control system for a wheeled vehicle comprisingmeans for generating a first electrical signal proportional to wheelacceleration,

means for producing a deceleration reference elec trical signal whichrepresents optimum wheel deceleration for vehicle braking on a highcoefficient of friction surface,

means responsive to the reference signal and the first signal forcyclically producing a brake control signal when said first andreference signals attain predetermined relationships,

brake pressure modulating means responsive to the brake control signalfor cyclically relieving and applying brake pressure, whereby the brakepressure modulating means relieves brake pressure to permit wheel speedrecovery and thereafter reapplies brake pressure,

and means responsive to positive wheel acceleration in each cycle foraltering the reference signal for the subsequent cycle to a lower valuewhen the positive wheel acceleration remains below a predetermined valueduring wheel speed recovery, so that the control system is conditionedto the value of wheel acceleration and hence to the coefficient-offriction between the wheel and the surface. 3. An anti-lock brakecontrol system for a vehicle comprising means for generating a firstelectrical signal proportional to wheel acceleration, means forproducing adeceleration reference electrical signal which during initialbrake application has a first value representing optimum wheeldeceleration for vehicle braking ona high coefficient of frictionsurface, I

wheeled means responsive to the reference signal and the first Ichanging the reference signal to a second value lower than the firstvalue when brake pressure relief is requested, and means responsive topositive wheel acceleration in each cycle for maintaining the referencesignal for the subsequent cycle at the second value when the positivewheel acceleration remains below a predetermined value during wheelspeed recovery,

and for changing the reference signal for the sub- 5 sequent cy'cle backto the first value when the positive wheel acceleration exceeds thepredetermined value during wheel speed recovery, so that the controlsystem is conditioned to the value of wheel acceleration and hence tothe coefficient of friction between the wheel and the surface.

4. An anti-lock brake control system for a wheeled vehicle comprisingmeans for generating first and second electrical signals proportional towheel acceleration,

means including a'logic circuit for producing a deceleration referenceelectrical signal which during initial brake application has a firstvalue representing optimum wheel deceleration for vehicle braking on ahigh coefficient of friction surface, the logic circuit contributing aportion of thev reference signal, means responsive to the referencesignal and the first signal for cyclically producing a brake controlsignal when said first and reference signals attain predeterminedrelationships,

brake pressure modulating means responsive to the brake control signalfor cyclically relieving and applying brake pressure,- whereby the brakepressure modulating means relieves brake pressure to permit wheel speedrecovery and thereafter reapplies brake pressure,

the logic circuit including means responsive to the brake control signalfor disabling the logic circuit output to change the reference signal toa second value lower than the first value when brake pressure relief isrequested,

and means including a signal level detector responsive to the secondsignal in each cycle and connected to the logic circuit for disablingthe logic circuit output to maintain the reference signal for thesubsequent cycle at the second value when the positive wheelacceleration remains below a predetermined value during wheel speedrecovery, and for enabling the logic circuit to change the referencesignal for the subsequent cycle back to the first value when thepositive wheel acceleration exceeds the predetermined value during wheelspeed recovery, so that the control system is conditioned to the valueof wheel acceleration and hence to the coefficient of friction betweenthe wheel and the surface.

5. An anti-lock brake control system for a wheeled vehicle comprisingmeans for generating a first electrical signal proportional to wheelacceleration,

means for producing a deceleration reference electrical signal whichrepresents optimum wheel deceleration for vehicle braking on a highcoefficient of friction surface,

means responsive to the reference signal and the first signal forcyclically producing a brake control signal when said first andreference signals attain predetermined relationships,

brake pressure modulating means responsive to the brake control signalfor cyclically relieving and applying brake pressure, whereby the brakepressure modulating means relieves brake pressure to permit wheel speedrecovery and thereafter reapplies brake pressure,

means responsive to positive wheel acceleration in each cycle foraltering the reference signal value for the subsequent cycle accordingto the value of the positive wheel acceleration during wheel speedrecovery, so that the control system is conditioned to the value ofwheel acceleration and hence to the coefiicient of friction between thewheel and the surface,

means responsive to wheel acceleration for sensing transient wheel hopsignals and for increasing the deceleration reference signal to renderthe brake control signal means less sensitive to the wheel hop signals.

6. An anti-lock brake control system for a wheeled vehicle comprisingmeans for generating a first electrical signal proportional to wheelacceleration,

means for producing a deceleration reference electrical signal whichrepresents optimum wheel deceleration for vehicle braking on a highcoefficient of friction surface,

means responsive to the reference signal and the first signal forcyclically producing a brake control signal when said first andreference signals attain predetermined relationships,

brake pressure modulating means responsive to the brake control signalfor cyclicalleilrelieving and applying brake pressure, whereby e brakepressure modulating means relieves brake pressure to permit wheel speedrecovery and thereafter reapplies brake pressure,

means responsive to the wheel acceleration in each cycle for alteringthe reference signal value for the subsequent cycle according to thevalue of the positive wheel acceleration during wheel speed recovery, sothat the control system is conditioned to the value of wheelacceleration and hence to the coefficient of friction between the wheeland the surface, and

means responsive to wheel acceleration including a bandpass filter forpassing transient wheel hop signals, and a signal level detectorresponsive to wheel hop signals of a predetermined minimum amplitude forincreasing the deceleration reference signal to render the brake controlsignal means less sensitive to the wheel hop signal.

1. An anti-lock brake control system for a wheeled vehicle adapted tothe coefficient of friction between a braked wheel and the surface onwhich the vehicle is supported, comprising means for generating a firstelectrical signal proportional to acceleration of a braked wheel, meansfor producing a deceleration reference electrical signal whichrepresents optimum wheel deceleration for vehicle braking on a givensurface, means responsive to the reference signal and the first signalfor cyclically producing a brake control signal when said first andreference signals attain predetermined relationships, brake pressuremodulating means responsive to the brake control signal for cyclicallyrelieving and applying brake pressure, whereby the brake pressuremodulating means relieves brake pressure to permit wheel speed recoveryand thereafter reapplies brake pressure, and means responsive topositive wheel acceleration in each cycle for altering the referencesignal value for the subsequent cycle according to the value of thepositive wheel acceleration during wheel speed recovery, so that thecontrol system is conditioned to the value of wheel acceleration andhence to the coefficient of friction between the wheel and thesurface.
 1. An anti-lock brake control system for a wheeled vehicleadapted to the coefficient of friction between a braked wheel and thesurface on which the vehicle is supported, comprising means forgenerating a first electrical signal proportional to acceleration of abraked wheel, means for producing a deceleration reference electricalsignal which represents optimum wheel deceleration for vehicle brakingon a given surface, means responsive to the reference signal and thefirst signal for cyclically producing a brake control signal when saidfirst and reference signals attain predetermined relationships, brakepressure modulating means responsive to the brake control signal forcyclically relieving and applying brake pressure, whereby the brakepressure modulating means relieves brake pressure to permit wheel speedrecovery and thereafter reapplies brake pressure, and means responsiveto positive wheel acceleration in each cycle for altering the referencesignal value for the subsequent cycle according to the value of thepositive wheel acceleration during wheel speed recovery, so that thecontrol system is conditioned to the value of wheel acceleration andhence to the coefficient of friction between the wheel and the surface.2. An anti-lock brake control system for a wheeled vehicle comprisingmeans for generating a first electrical signal proportional to wheelacceleration, means for producing a deceleration reference electricalsignal which represents optimum wheel deceleration for vehicle brakingon a high coefficient of friction surface, means responsive to thereference signal and the first signal for cyclically producing a brakecontrol signal when said first and reference signals attainpredetermined relationships, brake pressure modulating means responsiveto the brake control signal for cyclically relieving and applying brakepressure, whereby the brake pressure modulating means relieves brakepressure to permit wheel speed recovery and thereafter reapplies brakepressure, and means responsive to positive wheel acceleration in eachcycle for altering the reference signal for the subsequent cycle to alower value when the positive wheel acceleration remains below apredetermined value during wheel speed recovery, so that the controlsystem is conditioned to the value of wheel acceleration and hence tothe coefficient of friction between the wheel and the surface.
 3. Ananti-lock brake control system for a wheeled vehicle comprising meansfor generating a first electrical signal proportional to wheelacceleration, means for producing a deceleration reference electricalsignal which during initial brake application has a first valuerepresenting optimum wheel deceleration for vehicle braking on a highcoefficient of friction surface, means responsive to the referencesignal and the first signal for cyclically producing a brake controlsignal when said first and reference signals attain predeterminedrelationships, brake pressure modulating means responsive to the brakecontrol signal for cyclically relieving and applying brake pressure,whereby the brake pressure modulating means relieves brake pressure topermit wheel speed recovery and thereafter reapplies brake pressure,means responsive to the brake control signal for changing the referencesignal to a second value lower than the first value when brake pressurerelief is requested, and means responsive to positive wheel accelerationin each cycle for maintaining the reference signal for the subsequentcycle at the second value when the positive wheel acceleration remainsbelow a predetermined value during wheel speed recovery, and forchanging the reference signal for the subsequent cycle back to the firstvalue when the positive wheel acceleration exceeds the predeterminedvalue during wheel speed recovery, so that the control system isconditioned to the value of wheel acceleration and hence to thecoefficient of friction between the wheel and the surface.
 4. Ananti-lock brake control system for a wheeled vehicle comprising meansfor generating first and second electrical signals proportional to wheelacceleration, means including a logic circuit for producing adeceleration reference electrical signal which during initial brakeapplication has a first value representing optimum wheel decelerationfor vehicle braking on a high coefficient of friction surface, the logiccircuit contributing a portion of the reference signal, means responsiveto the reference signal and the first signal for cyclically producing abrake control signal when said first and reference signals attainpredetermined relationships, brake pressure modulating means responsiveto the brake control signal for cyclically relieving and applying brakepressure, whereby the brake pressure modulating means relieves brakepressure to permit wheel speed recovery and thereafter reapplies brakepressure, the logic circuit including means responsive to the brakecontrol signal for disabling the logic circuit output to change thereference signal to a second value lower than the first value when brakepressure relief is requested, and means including a signal leveldetector responsive to the second signal in each cycle and connected tothe logic circuit for disabling the logic circuit output to maintain thereference signal for the subsequent cycle at the second value when thepositive wheel acceleration remains below a predetermined value duringwheel speed recovery, and for enabling the logic circuit to change thereference signal for the subsequent cycle back to the first value whenthe positive wheel acceleration exceeds the predetermined value duringwheel speed recovery, so that the control system is conditioned to thevalue of wheel acceleration and hence to the coefficient of frictionbetween the wheel and the surface.
 5. An anti-lock brake control systemfor a wheeled vehicle comprising means for generating a first electricalsignal proportional to wheel acceleration, means for producing adeceleration reference electrical signal which represents optimum wheeldeceleration for vehicle braking on a high coefficient of frictionsurface, means responsive to the reference signal and the first signalfor cyclically producing a brake control signal when said first andreference signals attain predetermined relationships, brake pressuremodulating means responsive to the brake control signal for cyclicallyrelieving and applying brake pressure, whereby the brake pressuremodulating means relieves brake pressure to permit wheel speed recoveryand thereafter reapplies brake pressure, means responsive to positivewheel acceleration in each cycle for altering the reference signal valuefor the subsequent cycle according to the value of the positive wheelacceleration during wheel speed recovery, so that the control system isconditioned to the value of wheel acceleration and hence to thecoefficient of friction between the wheel and the surface, meansresponsive to wheel acceleration for sensing transient wheel hop signalsand for increasing the deceleration reference signal to render the brakecontrol signal means less sensitive to the wheel hop signals.